1. Field of the Invention
This invention relates to media sensors, and more particularly, to a sensor that uses the same integrated circuit to detect a signal as to produce it.
2. Description of Related Art
Optical sensors are often used in applications to determine the presence of a copy sheet or document passing through a certain point by providing a suitable signal in response to the copy sheet. Typically, the optical sensor includes a light source whose light beam is directed at the position at which the document is to be sensed. A light sensitive transducer, for example, a phototransistor or photodiode, is mounted in alignment relationship with the light source.
A recurring problem in printing machines is a false reading from a sensor due to not filtering out unwanted infrared light. Another problem is the contamination of optical sensors, particularly those in the paper path, by airborne toner particles, paper fibers, carrier particles, and other contaminants. These contaminants generally cause failure by coating the optical elements, thereby greatly reducing he illumination level at the sensor.
Another problem is degradation of optical sensors through aging of the light source with a corresponding decrease in light output in the sensing region.
U.S. Pat. No. 4,670,647 issued Jun. 2, 1987 to Hubble et al. is concerned with a self-adjusting document sensor compensating for degradation of the sensor system. A suitable light source and a detector are provided, the output of the detector being fed into an amplifier whose gain depends upon a feedback signal. Periodically, the output of the amplifier is compared to a reference. If the output of the amplifier falls below the reference, a pulse is sent to a ripple counter whose digital output is fed back to the amplifier to change the gain of the amplifier. If the detector is an unbiased photodiode operating in the transconductance mode, the leakage currents and their subsequent effect on output with amplifier gain changes will be minimized.
A media discriminating and media presence sensor is shown in U.S. Pat. No. 5,139,339 issued Aug. 18, 1992 that can detect and discriminate between paper and transparency uses a light emitting diode and two detectors configured so as to measure both diffuse and specular reflectivity of the media, and a media support surface that suppresses unwanted reflections.
Even though the above-mentioned prior art is useful, there is still a need, in printers for a sensor that lessens the risk of false paper readings.
Accordingly, pursuant to the features of the present invention, a sensor is disclosed that answers the above-mentioned problem by using two sample and hold circuits which are fired by the phase and antiphase components of an oscillator that also drives an infrared light emitting diode (LED). The output from a phototransistor that is optically coupled, via the paper to be sensed, to this LED is thus compared when the LED is both active and inactive thus filtering out unwanted infrared light that may cause a false paper reading.